1. Field of the Invention
This invention relates to a process of diffusing boron into semiconductor wafers with both low dispersion of sheet resistivity and low generation of lattice defects.
2. Description of the Prior Art
The use of boron nitride (BN) is known as a boron source in the diffusion of boron into transistor base regions. There is disclosed a process of diffusing boron into semiconductor wafers, e.g., silicon wafers, by using a sintered wafer-like disk of BN powder wherein the sintered BN is oxidized to B.sub.2 O.sub.3 (2BN+3/20.sub.2 .fwdarw.B.sub.2 O.sub.3 +N.sub.2), the boron oxide (B.sub.2 O.sub.3) being an active boron dopant (Japanese Patent Publication No. 28722/1968). The BN source contains, in the starting BN powder, impurities such as sodium phosphate, sodium oxide, iron oxide, calcium oxide or carbon, which originate in the production thereof. Further, since boric acid anhydride, calcium oxide, aluminum oxide, sodium oxide, aluminum phosphate or silicon dioxide (SiO.sub.2) are used as a binder for sintering, such binders would remain in the BN source.
To this end, one proposal has been made in which pyrolytic boron nitride (PBN) is used as a substitution for the sintered BN in an attempt to eliminate harmful effects caused by the above-mentioned impurities and binders (Japanese Patent Laid-Open Publication No. 101026/1987).
Another proposal has been made in which hydrogen injection is applied to an oxidized BN source to generate HBO.sub.2 which is an active boron dopant (presented by Dr. J. Stach et al at the 147th Meeting of The Electrochemical Society at Toront, Ontario, Canada on May 16, 1975). According to this proposal, since the vapor pressure of HBO.sub.2 is more than 10,000 times higher at 925.degree. C. compared to that of B.sub.2 O.sub.3, the former is far superior to the latter as an active dopant. With the high vapor pressure, a high-concentration atmosphere can be obtained at the same temperature, and as a result, it is possible to diffuse boron into the silicon wafer surface uniformly in high concentration. This tendency would remain even when the temperature varies, hence diffusion is possible even at a low temperature. However, yet in the above process applying hydrogen injection, if the usual sintered BN source is used, volatile suboxides would be formed from the impurities due to the reducing component, i.e., hydrogen, thus disadvantageously accelerating the contamination of the silicon wafers.
In an effort to solve the prior art problems, the present inventors previously proposed a process of diffusing boron into semiconductor wafers by placing a multiplicity of semiconductor wafers and pyrolytic boron nitride (PBN) dopant disks in a diffusion tube kept in an inert atmosphere at a high temperature and injecting H.sub.2 (Japanese Patent Laid-Open Publication No. 77118/1990). According to this process, although it is possible to remarkably improve dispersion of sheet resistivity (.rho.s) of the silicon wafer surface and also possible to eliminate the problems resulting from metallic impurities and carbon in the sintered BN source as well as other inconveniences due to the binder for sintering, lattice defects (i.e., stacking faults) of the resulting silicon wafers would be increased.